Virtualmin GPL (the ‘free’ version) and Virtualmin Pro differ in the following ways:
As you can see, Virtualmin GPL offers all the functionality that is required for a web presence provider to run a web hosting service. You can use Virtualmin GPL to manage your server(s) and client accounts and your clients can use Virtualmin GPL to carry out all the operations that one would reasonably expect from a web hosting control panel.
I found it a bit curious that you would think that only a pro version would let clients log in. Is there any other web hosting control panel that has such a limitation, based on which you would assume this to be true for Virtualmin too?
To answer your question, Virtualmin -> Virtual Server Summary: Administration username shows the username with which your client needs to login.
Your clients can access Virtualmin / Webmin via
clientdomain.tld:10000 or admin.clientdomain.tld or even virtualminhostname.tld:10000 or admin.virtualminhostname.tld
Your clients can access Usermin via
clientdomain.tld:20000 or webmail.clientdomain.tld or even virtualminhostname.tld:20000 or webmail.virtualminhostname.tld